This invention relates generally to data communications over public switched telephone networks (PSTN) 10 (refer to FIG. 3 for an exemplary system useful for implementing the present invention) and similarly operated private branch exchanges (PBX) and other like networks; and, in particular, to the high speed transmission of digital data using the voiceband frequencies over such networks.
Transmissions over the public switched telephone network (PSTN) 10 using voiceband frequencies are described in Townshend U.S. Pat. Nos. 5,801,695; 5,809,075; and 5,835,538 relating to xe2x80x9cHigh Speed Communications System for Analog Subscriber Connections,xe2x80x9d and Internet white papers of 3Com U.S. Robotics on x2(trademark) technology (http://x2.usr.com /technology/whitepapers.html) and of Rockwell Semiconductor Systems on K56flex(trademark) technology (http:// www.nb.rockwell.com/K56flex/whitepapers/k56whitepaper.html), the entireties of which (including entireties of all references cited therein) are incorporated herein by reference.
The feature that distinguishes x2(trademark) and K56flex(trademark) modem technologies from xDSL (digital subscriber line) technologies is its use of the line card 12 presently used in the local exchange 14 for digitization of voice as a data symbol generator. In this manner, a digital modem effectively sits in the local exchange 14 without any new equipment being placed in the exchange and with the local telco (telephone company) being unaware of the use of its line card 12 as a symbol generator. An important aspect of this idea is that the transmitter physically resides in the service provider""s building and that only the final conversion to symbols is done by the line card 12. If there is a lot of redundancy in the symbols, bandwidth will be wasted on the trunk. For instance, if the modulation requires an 8-bit symbol to transmit 6 bits of data then for every 8 bits transmitted over the digital telephony trunk there will be two xe2x80x9cwastedxe2x80x9d bits that do not contribute to the data rate of the modem. In the x2 technology, data rates of 56kbps are claimed for the standard digital telephone call rate of 7 bits (one bit often being lost to telco signaling) at 8 kHz. There is therefore no redundancy in the bits sent from the remote transmitter in the service provider 16, over the telco trunk digital network, to the line card 12. Therefore, without changing the line card 12, we cannot increase the xe2x80x9carithmeticxe2x80x9d capacity above 56 kbps. By arithmetic capacity we mean the capacity obtained by counting the number of bits per second used to generate the signal put onto the wire by the line card 12. The Shannon capacity of the wire is probably much higher, but to get closer to this capacity we require to transmit a signal onto the wire with more degrees of freedom than 56 kbps.
In order to increase the capacity further, we modify the line card 12 in the local exchange 14 to increase the arithmetic capacity of the system in a manner that minimizes the increase in cost of the line card 12. We propose increasing the sampling rate of the card 12 as a method for increasing the data rate of the modem. This method comes at a minimal cost as TI (Texas Instruments"") chipsets in the line cards 12 presently in use are capable of higher sampling rates and the data rate can be increased over band limited channels by using partial response techniques to send data symbols above the Nyquist limit.